There is a need to protect electric circuit elements from excessive current transients caused by load impedance changes or power supply surges. The prior art thermal fuse or electromagnetic circuit breaker opens the circuit at a predetermined current or current-time product. Such devices are slow acting and often do not respond to fast acting transients. Further, such devices typically must be manually reset or replaced after tripping. Other prior art approaches to limiting current have led to complex and often weighty circuit elements.
The incorporated reference in part teaches a magnetically biased saturable reactor transient current limiter that is fast acting, compact and which automatically resets upon return to a normal current level.
In all of the configurations disclosed by the incorporated reference, a biasing element is utilized to establish a predetermined constant magneto-motive force. As a result, such current limiters must always be operated with a sufficient bias level to accommodate any expected change in current through the electric circuit. As a result, relatively high energy is required to maintain this bias state to accommodate turn-on transients in high usage period operation as well as time periods when usage and transients are expected to be relatively low. In addition, certain direct current limiting embodiments described in the incorporated reference, such as that described in conjunction with FIG. 1 of that reference, are suitable for limiting current transients of one polarity. While such embodiments are effective with regard to such transients, it is often desirable to provide current limiting with respect to transients of either positive or negative polarity. Certain of the alternating current limiter embodiments of the incorporated reference, notably, those described in conjunction with FIGS. 10-13 of that reference, are suitable for use as limiting devices for such transients to a limited extent. However, in each of those configurations, a pair of magnetic material core elements are used with appropriately directed input coils to provide coupling to the electric circuit in which current is to be limited. The requirement for two such input core elements places substantial limits upon the size and cost and complexity of the configurations of the current limiters disclosed in the incorporated reference. Furthermore, these configurations are characterized by relatively large bias power requirements in order to maintain the appropriate flux bias in the input core elements.
It is an object of the present invention to provide a saturable reactor current limiting device which utilizes a relatively low level of bias energy.
It is another object to provide a saturable reactor bipolar current limiter device utilizing a relatively small amount of magnetic core material and associated windings.